Resolution of Generic Safety Issues: Issue 66: Steam Generator Requirements (Rev. 2) ( NUREG-0933, Main Report with Supplements 1–34 )
Following the steam generator tube rupture (SGTR) event at Ginna in January 1982,554 the staff proceeded to develop generic steam generator requirements which would help mitigate or reduce steam generator tube degradations and ruptures. To assist the staff, a supporting value/impact analysis was conducted by SAI.512 The SAI analysis was based on single SGTR events for a typical W design. The W design experience was believed to bound the experience of other NSSS vendors. Based on results of the SAI study and additional staff analyses, the probability of an SGTR event involved in a core-melt accident was determined to be 3.5 x 10-6 /RY.511 This low probability was the controlling factor in establishing the staff position that a SGTR is not of high risk to the public.
Given that the risk to the public from a SGTR is not high, the proposed staff requirements were assessed for potential reductions in occupational radiological exposure, potential reductions in SGTR frequency and tube degradation, and potential reduction frequency in forced plant outages.511 The proposed steam generator requirements are identified in Table 3.66-1.
Steam generator tubes are considered part of the RCS pressure boundary. Steam generator tube leaks, or tube ruptures, provide a direct path for the loss of primary system coolant through the steam generator (secondary system) and to the environment outside the primary containment structure. The two major safety implications are: (1) direct release of radioactive fission products, and (2) loss of RCS cooling water and ECCS water without the capability to recirculate the water as would be the case for LOCAs inside containment.
The possible solution to this issue is the implementation by licensees of those requirements identified as providing adequate mitigation and reduction of steam generator tube degradations and ruptures.
Based on the SAI analysis,512 four of the proposed requirements were recommended for implementation and assigned a high priority ranking: Item 66.5 combined with Item 66.6; Item 66.1.1; and Item 66.12. Four of the proposed requirements (Items 66.2.3; 66.2.4; 66.3; and 66.10) were redirected as Staff Action Items (See Issue 67). These items, if given further staff development, could result in more definitive requirements at a later date.
|Sub-Item No.||Requirement Title||Status*|
|66.1||Prevention and Detection of Loose Parts or Foreign Objects||-|
Inspection and Quality Assurance
|66.1.2||Loose Parts Monitoring System||DROP|
|66.2||Inservice Inspection Requirements||-|
|66.2.1||Full length (cold leg)Inspection||LI|
|66.2.2||72-Month Inspection Interval||LI|
|66.2.3||Supplemental Sampling||Covered in Issue 67|
|66.2.4||Denting Inspection||Covered in Issue 67|
|66.3||Improved Eddy Current Techniques||Covered in Issue 67|
|66.4||Upper Inspection Ports||DROP|
|66.5||Secondary Water Chemistry Program||NOTE 3(b)|
|66.6||Condenser Inservice Inspection||NOTE 3(b)|
|66.7||Standard Technical Specification Limit for Coolant Iodine Activity||LI|
|66.8||Primary to Secondary Leakage Limits||LI|
|66.9||Stabilization and Monitoring of Degraded Tubes||DROP|
|66.10||Reactor Coolant System Pressure Control||Covered in Issue 67|
|66.11||Containment Isolation and Reset||DROP|
|66.12||Safety Injection Signal Reset||NOTE 3(b)|
* For a description of the terms used for status, see Table II in the Introduction.
The 72-month inspection interval (Item 66.2.2) and the cold leg inspections (Item 66.2.1), which are sub-requirements of the ISI requirement (Item 66.2), are licensing issues as are the STS requirements on Iodine (Item 66.7) and RCS leakage (Item 66.8). These licensing issues will provide additional assurance that existing regulatory requirements on radiological releases will be maintained and further reduce the potential for the types of SGTR events which are not considered as part of the design basis. The remaining proposed requirements were dropped since they afforded little or no reduction in public risk and would likely result in additional ORE.
In general, the secondary ISI plus QA requirement (Item 66.1.1) is expected to eliminate up to 90% of SGTRs that can occur from loose parts in the secondary side. Operating experience has indicated that loose parts have accounted for approximately 50% of the SGTRs. The secondary water chemistry program (Item 66.5), in tandem with the condenser ISI (Item 66.6), is expected to reduce the remaining 50% of SGTRs and tube degradation by up to 70% in severely degraded units. The above 3 requirements (Items), in conjunction with improved eddy current techniques which will be developed further as a staff action item, offer the most effective value/impact ratios and best overall potential reductions (approximately 72%) in SGTRs. The SI reset requirement (Item 66.12) provides added assurance against a loss of defense-in-depth of the ECCS. The SI reset requirement will affect approximately 10 plants and the backfit costs are believed minimal. The program for resolving this issue was documented in SECY-84-13753 and SECY-85-62.765,766
In September 1988, USIs A-3, A-4, and A-5, which addressed steam generator tube integrity, were resolved and the staff's findings were published in NUREG-0844.681 The staff concluded that existing regulations provided the staff with sufficient authority to ensure that licensees were implementing steam generator programs that provided adequate protection to public health and safety. Issue 66 was based on possible steam generator requirements that were to evolve from the resolution of USIs A-3, A-4, and A-5. In view of the resolution of these USIs, the staff concluded that no new or revised requirements were necessary for Issue 66 since NUREG-0844681 addressed the safety concerns identified under Issue 66 (Items 66.1.1; 66.5; 66.6; and 66.12). Thus, this issue was RESOLVED and no new requirements were established.1147